HTLV-1 associated myelopathy (HAM) has been known to reveal various clinical manifestations of autonomic dysfunction; thus the focus of the present study was to investigate whether or not thoracic spinal cord injury frequently observed in HAM patients leads to cardiovascular autonomic dysfunction. The subjects consist of 18 patients in the chronic phase of HAM (group HAM) and 29 normal subjects (group C). They were examined for CVR-R and total heart rate. A power spectral analysis of R-R interval variability of their 24-hour Holter ECGs was also done. High frequency (HF) was an indicator of parasympathetic activity, while the low to high frequency ratio (L/H) was used as an indicator of sympathetic activity. Nominated HAM patients were evaluated by neurological examination using clinical indicators of HAM (EDSS, OMDS and FIM). To examine the degree of atrophy of the upper thoracic spinal cord, we performed individual MR (magnetic resonance) imaging of HAM spinal cords. Correlations between the above indicators and clinical indicators of HAM were examined. The L/H ratio was significantly lower in group HAM (1.67+/-0.79) than in group C. Significant reduction in the L/H ratio was observed in 8 (88.9%) of 9 patients in group HAM with orthostatic hypotension. The L/H ratio was significantly low in HAM patients with thoracic cord atrophy, strongly suggesting that the coexistence of a thoracic cord lesion is associated with a reduction of cardiovascular sympathetic activity. In conclusion, cardiovascular autonomic dysfunction in HAM patients is mainly associated with cardiac sympathetic efferent abnormalities in the upper thoracic segments.